The present invention relates to pressure compensating sound transducer and especially to an electromagnetic, high compliance, high displacement low frequency underwater sound transducer for use at great depths without gas compensation.
In the past, a great variety of sound transducers have been provided for use underwater and at varying pressures. Typically, the transducers are mounted through the hull of a ship, submarine, torpedo target, or the like, with a flexible sealing diaphragm sealing the sea water from the transducer but allowing the transmission of sound through the sealing diaphragm. The acoustic waves are generated by the transducer which may be an electromagnetic actuated piston actuated in response to electrical signals. A pressure differential exists between the sea water and against the sealing diaphragm and the interior of the hull supporting the transducer, requiring compensating or pressure release materials on the hull side of the transducer in order to permit pressure response of the transducer. In low pressure situations, soft materials such as air cell material, cork, air filled rubber and the like may be used behind the transducer to compensate for the pressure differential. However, for deep submergence and greater pressures, the hydrostatic pressure may exceed several hundred pounds per square inch which pressures impair performance, especially at low frequencies. In such high pressures it is common to use piezo electric transducers or to go to special compensating techniques for handling very low acoustic signals in electromagnetic transducers. These systems typically use a compressible gas which adjusts for the different pressures by compressing the gas to a smaller volume thereby balancing the pressure on both sides of the transducer diaphragm. At great depths and low frequencies this results in very large air filled chambers because of the great amount of compression with large pressures and thereby becomes impractical. The situation is sometimes handled by running air lines from the surface to the transducer and maintaining the pressure with pumps adjusted to the pressure for the particular depth that the transducer is operating. This, however, presents the problem of having to use long, flexible lines, along with high pressure pumps and maintaining the pressure adjusted for each different depth that the transducer is to be operated. It also requires a surface vessel to be located over the site of the transducer. Other techniques have been employed to deal with ambient pressures which vary between wide limits, such as when the transducers are subjected to a wide range of depths. Such techniques include the Toulis U.S. Pat. No. 3,277,433 and 3,274,537 and 3,021,504 directed towards compressing a plurality of air and gas filled tubes to compensate for the back pressure on the transducer. This system requires a large number of tubes and a large amount of space in order to compensate for great depths and low frequencies. At great depths, foam rubber or foam plastics collapse under hydrostatic pressure which may be equal to more than one thousand atmospheres of pressure. Also, the pressure release capability of air under such pressure is materially decreased if not eliminated since the density of air approaches out of the sea water environment. To compensate for such depths, various inventions have provided an air-tight system such as U.S. Pat. No. 3,277,434 which provides air-tight, air filled conical disc springs placed in close relation to the radiating surface from which it is desired to suppress the radiation and by having the spring require a relatively large amount of force to deflect air to a predetermined percent of its height. The Vincent U.S. Pat. No. 3,265,605 teaches an additional compliant tube for compensating for the hydrostatic pressure in an electromechanical tranducer similar to the technique taught in the Toulis patent. The Harris U.S. Pat. Nos. 3,018,466 and 3,108,247 teach a depth compensating transducer having a depth compensating reservoir. The Behrendt, et al, patent teaches a deep submergence transducer utilizing a collapsible diaphragm structure while the Wallen, et al., U.S. Pat. No. 3,501,741 utilizes mechanical spring biased pistons for pressure compensation. The Thompson U.S. Pat. No. 3,480,906 has the space between the inertia loading mass and the transducer element filled with a pressure transmitting compliant material while the Chatten, et al., U.S Pat. No. 3,296,583 is a hydrostatic pressure responsive apparatus utilizing a substantially cylindrical elastomeric envelope mounted in air-sealed condition on a spool-shaped frame to define a variable volume reservoir with the frame being formed with gas passageways to receive gas under pressure from the reservoir and to pass a gas to the closed chamber of the transducer. One prior art U.S. Pat. No. 2,978,672 to Barney for a hydrophone teaches a pressure compensating hydrophone connected to the rear of a transducer diaphragm using an automatic pressure operated valve for admitting fluid to the rear of the diaphragm whenever the pressure difference between the two faces exceeds a preassigned threshold and for withdrawing fluid from the rear face of the diaphragm when the pressure falls below a preassigned threshold.
The present system on the other hand teaches a solution to the pressure release of low frequency underwater sound transducers which permits operation without gas compensation by the use of pressure stiff chambers or hoses, or the like, with proper dimensions and compliance to provide the proper acoustic termination of the backside of a diaphragm of the transducer.